Overview of Membrane Structure and Function

Hey guys, today we're going to talk about chapter 5, membrane transport and cell signaling. So you'll notice I actually cut down the notes a little bit. We're not going to get too much into cellular signaling. We'll mostly be focusing on the structure and function of the membrane. So let's go ahead and get started with that. Okay, so life at the edge. We're talking about the edge of the cell. We're talking about the plasma membrane. Now remember that every single cell that has ever been or ever will be a cell has a plasma membrane that separates the inside from the outside. So it keeps the internal environment inside and keeps the external environment outside. Think about a water balloon. It is the squishy balloon part of the water balloon. It's what separates the cell from its environment. And every single cell has a plasma membrane. Now the plasma membrane is selectively permeable. This means it's choosy. It gets to decide what things are going to come in and what things are going to go out. Okay, so it's getting to select what it is. permeable to. Okay, so that's where that term comes from. It's getting to choose what comes in and what goes out in order to keep the cell in homeostasis. That's the fancy word for balance. This is kind of like a molecular structure if you were to look at like a like a filled molecular structure of the cellular membrane. You'll see that all those like little red and gray or red and white areas, that's water. Okay, so we're going to talk about how the membrane is both hydrophilic and hydrophobic. So you can see that the exterior and the most interior portion of the cell membrane is going to be interacting with water. Okay, and then the innards are not so much unless you're dealing with those blue sections there, which I'm assuming are going to be our aquaporins, which transport water. It's a specific protein that's designed to do that. Okay, so let's talk about the actual setup. What is this squishy balloon of our cells made of, right? So all plasma membranes are made of phospholipids. That's why the plasma membrane is also called the phospholipid bilayer, okay? So that makes phospholipids the most abundant lipid in our cell membranes. Well, that makes sense. It's called a what? It's called a phospholipid bilayer. It is literally two layers of these fancy fats called phospholipids. It's a... fatty acid, two fatty acid tails with the phosphate group slapped on the top. Okay. And these phospholipids are very specific in structure that I just explained. And it gives them these certain properties and it's called being amphipathic, which means that they have both a hydrophobic, water-fearing, and hydrophilic, water-loving region. So you have, it's kind of like a little lollipop with two tails, two sticks. So the lollipop part is going to be your phosphate head. We'll look at a picture of this in a minute. Okay, that is the part that is the polar head. So it will interact with water because water is also, you know, polar. Okay, and then the hydrocarbon tails are going to be the hydrophobic region, which will actually form the center of our bilayer membrane, which again, we'll look at some pictures in a minute to make sure you have the full on visual for our cell membrane. Okay, that is the most like stable boundary that we can have for ourselves because we have aqueous solutions both inside and outside of ourselves. So it's important that we have those phosphate groups that are polar heads that are going to interact with the water while still protecting our hydrocarbon tails that are going to be on the interior of the membrane. So this is the overall structure that I'm talking about. So you see this like little wavy guy here. It's all these like little gray or white phosphate heads with the two little yellow tails coming off of each one of them. So those are our amphipathic phospholipids. So this is a phospholipid by layer, by for two layers, because you have a layer on the top and you have a layer on the bottom. And they come together and kind of make like a hydrocarbon tail sandwich. Okay. But you don't just see phospholipids here. You see all kinds of other stuff, right? There's a whole system of things happening here. You have these globular protein kidney bean looking structures. Okay, those are going to be your proteins. And notice that you have some that are called integral proteins, which are integrated. It's funny how words work together there. Integral proteins that are integrated into the cell membrane. And you also have peripheral proteins that are on the periphery of our cell membrane. Notice that they are on the interior though. Okay, so you have different proteins that are found there. You also have glyco. proteins. Glyco is a prefix that means sugar. So these are just sugars that are slapped on top of a protein. So you can see that those are outlined in this picture. You also see glycolipids. So again, sugars that are attached to lipids. You just see sugars themselves like our carbohydrate chains that are extending from the cellular membrane. You also see these fibers that make up our extracellular matrix, which gets really complicated with all of the things we talked about in our last unit with the microtubules, microfilaments, intermediate filaments, all of those things to help us, you know, maintain structure and help to transport things around our cell and also anchor cells to each other and things like that. You also have these little guys embedded inside of the membrane itself called cholesterol. So remember that cholesterols are steroids, which... Some people think are proteins, but that's false. They are specialized lipids. So it's a lipid bilayer. When in doubt, it's a lipid. Okay. So these cholesterols are in there to help prevent the hydrocarbon tails from the phospholipids from packing too tightly together because they have these like big, you know, ring structures. You also have like the cytoskeleton that you see on the internal side there, the cytoplasmic side of the cell membrane, which again are going to help to... you know, anchor some of our organelles. They're going to offer structural support for our cell as it is, right? So we have a lot of different players here, but notice like out of our biomolecules that we've talked about, there's one that I didn't mention. It's a nucleic acid. Spoiler alert. Okay. What is a nucleic acid? A nucleic acid is DNA. You're right. DNA. DNA is where? It's inside the nucleus, right? It's inside the nucleus, which has nothing to do with the cell membrane. OK, the cell membrane is the thing that interacts with the environment. You do not want your DNA interacting with the environment because it can easily be damaged. And we don't need that because that's the directions to make everything else that that organism needs. So we can't we have to protect that. It's kept somewhere else. OK, so it's often asked on exams, quizzes and so on, which of the following biomolecules is not present in the cellular membrane? Your nucleic acids, it's going to be nucleic acids, okay? Because that's your DNA. If your DNA is in your cell membrane, you've got a bigger problem, okay? So let's keep going here. So our membrane proteins, we talked about the lipids being amphipathic. A lot of our membrane proteins are also amphipathic because the ones that, like, span or go all the way through the membrane have to kind of have a hydrophilic and hydrophobic region as well. So you'll see that the hydrophilic... portion is what's going to protrude or extend beyond the internal side because think about it, the outermost part and the innermost part are going to be interacting with water. So those have to be water loving. Whereas the middle section, like those yellow hydrocarbon tails, those sections are all going to be hydrophobic because they're not interacting with water at all. Okay, so keep that in mind too. So our phospholipids and our proteins are both amphipathic, which means that they have hydrophobic and hydrophilic regions within the same molecule. Okay. Our cell membrane is also called, we have like, it follows this rule called the fluid mosaic model. Okay. So what does that even mean? Okay. So it's fluid. Okay. So that means it's not rigid. It's flexible. That's how I like to think about it. Mosaic has a whole bunch of different pieces. Well, look at this. Obviously it has a whole bunch of pieces. Okay. And then a model. Well, that's easy. It's a model. Okay. So fluid mosaic model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. So essentially, you see this picture here. These things do not have to stay where they are. They're very flexible. They're very loosey-goosey. They can move around. Okay, our phospholipids can shift around. The proteins can move laterally, which means side to side. Okay, throughout the membrane. So they're not just stuck in these positions, rigid robots. Okay, we are not in a cell wall. We are in a cell membrane, which is a lot more fluid and flexible than a cell wall. Okay, so all of these components can kind of move around and do what they need to do and go with the flow. It's kind of chill, relaxed. Okay, it's not this rigid, like one, two, one, two. It's very fluid. That's why it's a fluid mosaic, lots of pieces model. Makes sense. Okay, and we have groups of certain proteins or certain lipids that can associate with long lasting interactions or short term interactions, just depending on the different functions that they need to accomplish. So sometimes like especially like our peripheral proteins, you'll see that they move a lot. Our other integral proteins or transmembrane proteins, they're going to shift through the cell membrane as well. And then when we talk about like vesicular transport, like obviously, we're going to be introducing new components to our membrane, of course, still dealing with phospholipid bilayers, but we're going to be introducing possibly new receptors or more phospholipids and things like that the membrane is constantly changing. Okay, so that's Keep in mind all of that when you think about the fluid mosaic model. So here's a closer up image of our phospholipid bilayer here. This is a much simpler structure because it doesn't include all of the rest of the mosaic. It doesn't include the proteins and the carbs and all of that. So here you can get a better image of just a phospholipid bilayer and where exactly our hydrophilic and hydrophobic portions are. Remember that our phospholipids have a phosphate head that is a polar phosphate head so it will interact with water which makes it hydrophilic. Whereas the hydrocarbon tails, which are like the yellowy orangey structures here, are literally just made of hydrogen and carbon. They are hydrophobic. So that would be the area that does not include water. It is the interior of our cell membrane or a plasma membrane. So this is what I was talking about, the fluidity of our membrane. So a lot of the lipids and some of the proteins can move laterally. So like side to side. OK. So the lateral movement of our phospholipids is really rapid. They're kind of like always like in flux. They're always moving around. Proteins are a lot bigger, okay? And there's phospholipids in between all of our proteins. So that movement is a whole lot slower. Think about these little tiny, tiny phospholipids that are just flipping around doing whatever they need to do, okay? And then our proteins that are much larger are going to have, it's kind of like trying to walk through like a crowded room. Like they're just bumping into all these other phospholipids. So their movement is going to be slowed down a lot. And some of our proteins move in a directed manner while others are more anchored in place. And this has to do with like the extracellular matrix and also the cytoskeleton on the internal side of our membranes. Sometimes you have proteins that are anchored in certain positions and sometimes you have proteins that are moving. That's more of like our peripheral proteins and things like that. They have directed movement that says you move from point A to point B and then you stop and go back again. Like it's something that is controlled in the cell. We're also going to talk about these little guys called cholesterol and how they work with our different temperatures and the fluidity of our membrane. So as temperatures cool, just in general, you know that that kind of slows things down. OK, membranes switch from a fluid state to a solid state. You know that you don't want your cells to be frozen. Right. You don't want the membrane to become a complete solid. OK. So the temperature at which the membrane solidifies depends on the types of liquids, both inside the cell, outside the cell, and anything that might be present within the cell membrane itself, just depending on the organism. The membrane remains fluid to a lower temperature if it's rich in phospholipids with unsaturated hydrocarbon tails. Now remember, unsaturated means that that lipid does not have all of the hydrogen it possibly can. This kind of lipid has our double bonds, our carbon to carbon double bonds. So it's bent, which prevents them from packing very tightly together. So let's look at this point again. A membrane remains fluid to a lower temperature if it is rich, so has a lot of phosphates, I'm sorry, phospholipids with unsaturated hydrocarbon tails. If they're all bent and funky angles and everything, you can't pack them all together. That's why like olive oil is a liquid at room temperature. Also, if you put it in your fridge, it's not going to become like a solid. It's still going to kind of be like a little bit more of like a gel, right? So as you cool down cells that have a large concentration of unsaturated hydrocarbon tails, the tails still can't pack very tightly together. So it's going to maintain fluidity, which is a good thing for that cell in order to maintain homeostasis with its environment. The whole point of having a cell membrane is to interact with the exterior of the cell, right? You control what comes in and what goes out of the cell. I need to bring in food particles. I need to get rid of waste particles. These things are important to keep a cell alive and the cooler temperatures. You know, a cell still needs to live. Right. So it's important to maintain the integrity of the membrane in various temperatures. Membranes must be fluid to work properly. That's what I was just saying. And it's comparing salad oil. Think about it like olive oil. Right. Which is what I just explained. So this is another image just to kind of like show you what I was just talking about. The more unsaturated hydrocarbon tails you have, the more spaced out your phospholipids, which just means that you have more fluidity, okay? So you have a very fluid membrane up at the top, and then you have a more viscous, which means that everything is packed a lot tighter together. And then you have these little guys called cholesterol that we threw into the mix. Now remember, I said cholesterol are a fancy type of lipid. It is called a steroid, okay? So cholesterol reduces the membrane fluidity at moderate temperatures. But, but, right, it reduces the membrane fluidity at moderate temperatures, comma, however, right, at low temperatures, it's going to prevent the membrane from being solidified, which is very important for life. Because if your membrane is solid, you're probably not going to be alive for very long. Okay. So that's why cholesterol, I always say that they reduce membrane fluidity, or I'm sorry, that they increase membrane fluidity. Well, it's true, especially at low temperatures. Okay, but here it's saying that at moderate temperatures, it reduces the fluidity. Well, right, the phospholipids can't move around as easily if you have this cholesterol in there. However, the cholesterol is vital to make sure that your cells function at cooler temperatures. Okay, so that's what I was just kind of explaining there. So we also have variations in the lipid composition of our cell membrane, just depending on the environment of the organism that the cell is present in. So like I said, cholesterol is also a lipid, fossil lipids are lipids, so you're going to have different concentrations of these two different lipids and among other types of like glycolipids and things like that that are present in the cell membrane, depending on the type of organism and depending on the environment that it lives in. The ability to change the lipid composition in response to a temperature change has evolved in organisms over time. So this is something that we have evolved over time. So there's organisms called psychrophiles that live like inside of ice caps. There are these little tiny bacteria that live inside of ice caps. They have a whole boatload of cholesterol. OK, that is an adaptation for that specific environment. And that is a great example of evolution if I've ever seen one. Okay, so a membrane is a collage of different proteins as well, right? We've been talking about lipids. You have all these different kinds of lipids and they're really important, fantastic, wonderful. Now let's talk about the proteins. There's a whole bunch of different types of proteins. The main ones are going to be your integral proteins and your peripheral proteins. They often work together and so they're kind of clustered together and they're embedded in the fluid matrix of the lipid bilayer. Or like I said, in the case of peripheral proteins, they're kind of like rolling along the internal surface. So these are the two proteins I was just talking about. So the integral proteins, remember that they have both hydrophilic and hydrophobic regions because they span the whole membrane. They are transmembrane proteins. They go all the way through it. Okay, so when the integral proteins penetrate the hydrophobic interior of the lipid bilayer, they go all the way through it. Okay, so they span the membrane, and that's why they're called transmembrane proteins because trans just means like across the membrane, and then it's a protein. Okay, so the hydrophobic regions of the integral... proteins consist of one or more stretches of nonpolar, so it's not going to interact with water, amino acids, okay? These are often alpha helices. Now recall that alpha helices are the secondary structure for proteins, the primary structure being the string of amino acids held together by peptide bonds, okay? So these integral proteins, because they span, they go all the way through the membrane, they have hydrophobic regions. that contain nonpolar amino acids. That makes sense because the polar regions would interact with water. So the nonpolar regions would be on the interior side of, like in the middle of our bilayer. Our peripheral proteins are loosely bound to the surface of the membrane, and you typically see them on the internal surface, so like on the cytoplasmic side, and literally think about like a log roll. That's kind of what they do. They like roll along the cell surface. This is an example of a transmembrane protein. So recall the structure of an amino acid that you have an amino group that has a nitrogen in it and you have a carboxylic acid group that has a carbon in it. That's the N-terminus, the nitrogen side with the amino group, and the C-terminus with the carboxylic acid group. But of course in between the N-terminus and the C-terminus we could have hundreds to thousands of different amino acids that make up this particular protein. But you'll see that there's a whole bunch of alpha helices and a couple beta sheets up there at the top. I don't know if you can see the mouse moving in these recordings but here we go I'm going to try. These are like the beta sheets here and then here you have the alpha helices that go all the way through the membrane. So these residues here, these amino acids that are present in these regions are going to be our non-polar ones. Whereas the ones on the surface here and the cytoplasmic side here are going to be more polar because they're going to have the potential to interact with water on the inside and outside of our cell. So in keeping with our theme of proteins here, proteins have a whole bunch of different functions. OK, our membrane proteins can function to do these six different things. They can be transport proteins. They can have enzymatic activity. They can be attached to our cytoskeleton and the extracellular matrix. They can be involved in cell-to-cell recognition. They can help with intracellular joining. And they can also help with signal transduction. So let's take a look at those. These are pictures to kind of like, you know, display each of these different characteristics. So transporting molecules, you see passive and active transport there. Passive would be on this side. Again, I hope that you can see the arrows. with the blue molecules from high concentration to low concentration without the use of ATP. This is a protein channel. Over here you have a carrier protein or also called a solute pump. You have the input of ATP which is going to tell you that this is active transport moving from a low concentration to a high concentration. You also have enzymes. So you have enzymes that are embedded here that are going to have substrates and products and substrates and products. So it looks like our initial substrate here is this little guy here and then we have a terminal product here. Okay you also have attachments to the cytoskeleton extracellular matrix. This is in order to keep things anchored. You also have cell to cell recognition. We use this especially in our immune system that our white blood cells can tell what cells make up our bodies versus what cells do not. So for instance you know if you inhale someone's snot with all the bacteria or virus or whatever is in their body and they're sick or something that's disgusting in the first place but that's how it works. Okay, then your cells would hopefully identify that something has a foreign invader tag, which would be like these carbohydrates that are extending from the cell membrane. If it's something that the other cell would recognize then it's obviously part of that organism. Whereas if it was a foreign invader, it would not recognize this carbohydrate chain and it would probably trigger some sort of phagocytosis in order to destroy it. Okay we also have an intracellular joining so how to connect cells together and then signal transduction. So you have a receptor on the outside of our membrane and you have some sort of signal molecule perhaps like a hormone that's going to dock and bind to the receptor and then cause some sort of like domino effect some sort of chain of reactions to occur within the cell. So we're going to talk about cell-to-cell recognition first. So the role of our membrane carbohydrates, like I just talked about in this image here with these little green carbohydrates, is for cell-to-cell recognition. So cells recognize each other by binding to the surface molecules, and so the things that are extending from the cell itself, in this case carbohydrates, okay, most of the time it's going to be carbohydrates. So you have like glycolipids and glycoproteins that have these carbohydrate chains that extend. It's kind of like waving your hand around. as a cell, right? Like, hey, look at me. I'm here. I'm supposed to be here. I am a self-cell, okay? It's how they identify each other. So that's on the extracellular matrix. I'm sorry, that's on the extracellular surface of the cell membrane, obviously. So then other cells would like bump into that and be like, oh, hey, what's up? You're supposed to be here. Cool. Have a nice day. Okay. And then you have your membrane carbohydrates. They're going to be covalently bonded to the lipids to form glycolipids. Or more commonly, glycoproteins, which we've talked about previously already. Okay. And the carbohydrates on the external or the extracellular side of the plasma membrane vary among species and individuals. Like I said, if you have something inside of your body that is not part of your body, your cells know it's not supposed to be there. Okay. And that's actually an area of research right now. And so try to come up with like different drugs to help treat like antibiotic resistant bacteria and also develop vaccines and things like that. So it's a huge topic in research right now. So our membranes have sidedness, right? They have an internal side and external side. We've been talking about a lot of things that are happening on the external side, like the cell to cell recognition and our transmembrane proteins that span both sides. Okay. So it's an asymmetrical distribution of all these things that we've been talking about. If you click all the way back to the, you know, like second or third slide where I have that picture that has like the the purple proteins and all of that you'll see that it's not even on either side but that's important because like i said with the cell to cell communication and all that you have to have that all on the outside because if it's on the inside then how are the other cells supposed to pick it you know what i mean that just doesn't make any sense okay so it is built that way because we have the er and the golgi apparatus that are helping to like synthesize our proteins and our lipids and then package them and send them wherever they're supposed to go so they get like these little kind of like a zip code like when you send mail it's You put your zip code like I live in Kingwood 77339, right? Like that's going to be Like my zip code it helps narrow down where I am located And so that's what's put on to these glycolipids Glycoproteins and other like transmembrane proteins and everything that get exported to the cellular surface They have these you know little zip codes to kind of identify like oh where are they supposed to go on the inside on the outside? All the way through the cell membrane. Let me know. Okay, that's how we can label them and it's all of course processed by the ER and the Golgi apparatus. Okay, so here's a picture of that happening. So you can see that you have our ER, okay, where you have these vesicles that are leaving that hold, in this case, it looks like we have some glycoproteins and we have some glycolipids, okay, and then they're going to be packaged and further modified by the Golgi apparatus to be identified as going to the exterior surface. In this case, we also have that glycoprotein. that is going to be a transmembrane protein here. So the Golgi apparatus is going to modify and like slap those zip codes onto our products here in order to send them where they're supposed to go, just like mail. Okay, so talking about the cell membrane, we've talked about the different components and how they all work together and that it's very fluid and it's a lot of lipids and it's a lot of proteins, but absolutely zero nucleic acids. Okay, so like what's the point of all this? We've talked about the selectively permeable membrane. The whole point is to regulate what is going in and what is going out of the cell. So it is going to regulate the transport of substances across its own membrane. Okay, so this is where our molecular traffic is happening. And again, selectively permeable means that it can control, it chooses. These things can come in. These things cannot come in. So we have things that are like hydrophobic, nonpolar molecules, like our hydrocarbon tails of our lipids, okay? These can dissolve in the lipid bilayer because the lipid bilayer has that interior region that is hydrophobic. So these things can cross the membrane very easily, okay? Things that are polar. that have these, you know, slightly negative, slightly positive regions that like to be around water cannot just go through the membrane. They need assistance to do that, right? Can't just go through a hydrophobic area if it is polar. Has to be only non-polar things that can just easily just slip on through the membrane, whereas our polar molecules and larger molecules cannot. So we have things called transport proteins. We said that proteins help us transport things in and out of the cell. They absolutely do. So transport proteins allow the passage of our hydrophilic substances across the membrane. Because remember, they can't just go straight in between two phospholipids. It actually has to be transported through a protein because it is hydrophilic. It cannot cross through the hydrophobic region on the inside of our bilayer. Okay. So some of our transport proteins can be called channel proteins. They can also be called like carrier proteins. Okay. But these channel proteins have a hydrophilic... channel that allows certain molecules and ions to use that tunnel for transport. Okay. And our channel proteins that are specifically called aquaporins help to move water because water is a polar substance, right? So we need to have a, um, we need to have a, a channel protein, a transport protein that's going to allow water to move in and out of the cell. Very important. Water is a huge part of life. Okay. So those particular proteins are called aquaporins. I think about like water pores because that's what they are. Okay, we have other transport proteins called carrier proteins. These help to shuttle things back and forth across the membrane. And typically, in order to do this, they're going to change shape. Whereas more of like our channel is just kind of like a direct route. It's just like a slip and slide in and out. Okay. Whereas our carrier proteins physically change shape, and they typically require the input of energy. Okay, so we're going to talk about passive transport first. So passive transport is the diffusion of a substance across a membrane with no energy. So our cellular energy is called ATP. So passive transport does not use any ATP, okay? So an example here is diffusion. So the diffusion is the tendency of molecules to spread out evenly into an available space. For instance, I stand at the front of the classroom, I spray a whole bunch of scented Lysol because some of y'all stanky after gym class, if you know what I mean. Okay, I stand at the front of the room and I hose the ish out of my classroom. The people sitting right in front of me are going to be coughing and gagging. Sorry, front row. Right. But eventually, all the way in the back, those people will finally be able to smell it, you know, maybe like 10, 15 minutes later. Okay, that's diffusion. And diffusion is passive transport of molecules from an area of high concentration, which is coming out of the Lysol nozzle. to low concentration, which would be all the way in the back of my room where I was not standing. It's the furthest point away from me. Okay, that would be a very low concentration of whatever I'm spraying. Okay, so there's an example of diffusion for you. Okay, so all these molecules are moving around randomly. Okay, but remember that temperature affects diffusion. Okay, so as you increase the temperature, the molecules are going to be moving faster, which will increase the rate of diffusion. So think about organisms that live in hotter or colder environments and how that would impact them, right? Okay, um, so we have something called a dynamic equilibrium, which is what we're going to be focusing on here. So dynamic equilibrium is when you have like obviously equilibrium. It means equal, it means even, right? But a dynamic equilibrium is when you have like the same concentration on the inside of the cell and the outside of the cell. But it doesn't mean that everything stops moving like oh, it's even now time out stop everyone holds still. That's not what that means, okay? You have things that are constantly moving in and out of the membrane and they're always in motion, but it's happening at about the same rate. So then the concentration doesn't change. If I have 10 molecules of, you know, oxygen on the inside of the cell and 10 molecules on the outside of the cell, that doesn't just mean, all right, time out, oxygen, stop doing your thing. Like you just, just stay there. Everyone holds still. We're done. We reached equilibrium. We're finished. Wash our hands of it. We're done. That's not what that means. Okay, so the oxygen is still going to be coming and going, but it's going to be doing that at an even rate. So it's not like suddenly we have 20 molecules on one side and zero on the other. Okay, it's going to be going in and out at the same rate. That's what our dynamic equilibrium is. Okay, so here's an example of this diffusion that we just talked about. So we have water and we have molecules of dye. And you'll notice that there is a selectively permeable membrane, the dotted line. Okay. So these molecules of dye are obviously put in here on the left-hand side, and they're going to bounce around, and they're going to bounce around, and they're going to bounce around. So eventually a couple of them are going to bounce around over here, and then they're going to keep bouncing around, and a couple more are going to get pushed over here. But does that mean that when they reach an equal equilibrium, an equal amount on either side of this foam membrane here, does that mean that the molecule is like, oh, stop, everyone, we're here, you have arrived. No, they're going to keep bouncing around and doing their thing because they're molecules. They don't got no laws. Okay. They're going to keep doing their thing. They're going to be bouncing around. Okay. And they're going to be moving in and out back and forth. And like, if like one or two extra are on one side of the membrane, it's not going to, it's okay because it's going to equal itself back out. It will reach that dynamic equilibrium again. Okay. Same thing is happening down here, but like this one's a little bit fancy. It has both yellow and purple molecules in it. So it's showing you that the direction of diffusion is still from left to right for a little yellowy orangey guys. Our purple guys are going to be moving from the right side to the left side. You'll notice that, again, they do reach an equilibrium, but that does not mean that it stops. That's one huge misconception in biology as a whole. Okay, these molecules are still going to be moving in and out. And if it's unequal for like a little bit of time, that's okay. It's going to equal back out again. Like that's just life, literally. Okay, so substances tend to diffuse down or with their concentration gradient. from where there is more concentration to less concentration. So where we have a high amount of that stuff, whatever the stuff is that is diffusing, to where we have very little or none of that stuff that is diffusing. Okay, so no work has to be done. No ATP needs to be hydrolyzed. There is no energy here for substances to move down their concentration gradient. It just naturally flows that way. Think about a bowling ball. You push it up a hill, okay, that's going to require energy. You get to the top and let it roll down, there's no energy there. That is passive, okay? We're letting it roll at this point. Okay, so that is what passive transport is because there is no energy utilized by the cell in order to make this reaction happen. Our next kind of passive transport is going to be osmosis. So osmosis is the diffusion of water and specifically water. If it says water, it's got to be talking about osmosis because that's how it happens. Okay. Please don't bring up osmosis Jones. It's like a triggering point for me. And everyone's like, let's watch that movie. It's no, isn't it about some kid getting sick, which is not about the movement of water in and out of cells. Anyway, osmosis is the passive transport of water through a selectively permeable membrane. And again, it's still going to follow the same directions. It's going to be from a high concentration of water to a low concentration of water. Okay, so it will diffuse through a membrane from where you have a lot of water to where you have less water. Okay, so in this case here, this is called a U-tube. And why is it called a U-tube? Because scientists are creative. No, I'm just kidding. It's a U. That's why. Okay, not like YouTube, like the thing you're watching this on. No, it's literally the letter. Okay, so you have water. both sides of the semi-permeable membrane, but then you also have sugar. Sugar is a solute in this situation. The water is the solvent, the solute is the sugar, and all together it makes a solution. Okay, so our semi-permeable membrane in the middle is permeable to water but not sugar, which means sugar cannot pass through that membrane. So in order to reach our dynamic equilibrium, water's got to move. That's called osmosis. Okay, so if you look... at the beginning picture here, we have fewer sugar particles here and we have more sugar particles here. Well if there's fewer sugar particles here that means there's going to be more water here. high concentration of water to a low concentration of water comparatively. That's why the water goes up. Okay. It's going to move from the left side to the right side. Okay. I like to think about the solute, whatever it is. It could be sugar. It could be salt. It could be iodine. It could be anything. Okay. Whatever the solute is, I think about it as salt because salt water is something easy for everyone's brains to understand. Wherever there's more salt, Also called stuff or solute. Whatever it is. Pretend it's salt. Wherever there's more salt, salt sucks. What does it suck? It sucks water towards it. So if there's a lot of, let's call this salt because it's the sugar, it's the solute, it's the stuff. There's more salt on this side. Salt sucks the water towards it and oh my god, look at that, the water increased on this side. That's how I like to think about it too. So the definition of osmosis says that water moves from an area of high concentration to an area of low concentration. Okay, but I like to think about it like simpler terms. Where is there more salt? Salt, it sucks the water towards it. Simple. Okay, very, very simple. So when we're talking about these solutions and the amount of salt, stuff, solute, whatever that they have in them, right? It's called tonicity is what we're really dealing with. So tonicity is the ability of a surrounding solution to change the cell. It's either going to gain water or lose water. So we're talking about isotonic, hypertonic. These are tonicities, okay? It's the ability of that solution to affect what's going on with that cell water-wise. So in an isotonic solution, very grammatically correct, I'm going to tell you that I so happy. A cell is so happy when it is in an isotonic solution because the solute concentration is the same on the inside of the cell and the outside of the cell. This means the salt outside and inside is equal. Salt can't suck to one direction here because it's equal. So what does water do? Does it hold still? No. This is dealing with a dynamic equilibrium. So the water is going to move in and out at an equal rate. And this is where the cells are happy because they're not changing in size. Nothing bad is happening to them. This is where they would like to function. Okay, dealing with a hypertonic solution. Hyper, give a little kid a lot of sugar, they are hyper, they have higher energy. Hyper is high amount of solute. Again, I'm gonna use the word salt, because salt water, am I right? It's very simple, okay? In a hypertonic solution, let's say that you have the ocean. Great, it's salty. You throw some cells into the ocean. Great, hypertonic solution because there is more salt. inside of the ocean than there is inside of those cells. Higher salt is hypertonic. What's gonna happen to that cell? Look at the word hyper. This little R right here, you're gonna think wrinkled like a raisin. Now, wrinkled starts with a W, but English is hard, so let's get over it. Hypertonic, R, wrinkled like a raisin, okay? If you're wrinkling up like a raisin, you're shrinking. Why? Where's the water going? It's going out of you. The salt. in the ocean is sucking the water out of that cell. Okay. Like not literally, but we're going to think about it like that because it's very easy. Okay. Hypertonic solutions have a high amount of stuff, salt. Okay. And it's going to suck the water out of a cell. And if the water is leaving the cell, think about a balloon, a water balloon, you let the water out. Only get what happens to it. It gets smaller. It shrinks called cremation. Okay. Hypotonic solutions. Hypo, hippo, big fat O. Okay, so this one was R, wrinkled like a raisin, and I'm telling you hypo, hippo, big fat O. What's gonna happen to that cell? It's gonna get huge because there's more salt on the inside of the cell. All the water is going to go towards that. It's going to get huge. And in some cases, it will blow up on a cellular level, which is called lysis. Okay, so this is when you have less solute in the surrounding solution. which means that all of the salt is going to be inside of the cell, then the water is going to go towards that salt, which would be inside of the cell. And as water rushes in, it's going to cause the cell to pop, just like overfilling a water balloon. So those are three different types of solutions and understand it has to be in relation to something. You can't have a beaker of water in front of you and say, that's an isotonic solution. You don't know. Is there a cell in there and you're watching it be happy and healthy and living its best cellular life? No. you have to have it in relationship to something else. You have to have a cell inside of the solution before you can judge. Is it iso, hyper, hypotonic? It's a relationship between two different solute concentrations, two different solutions. So please keep that in mind. So animal cells and plant cells behave a little bit differently in these different environments. So we have a hypotonic solution, hypo, hippo, big fat O. What's happening? Water's going in. Look at our cell. It is very unhappy. It is lysed, which means it is broken open, not like lice. bug in your hair that makes you itchy. Okay. Isotonic, again, grammatically correct. Isohealthy. This is where a cell wants to be. Okay. This is a red blood cell. It's doing its best life over here. This is a normal environment. Okay. We have water going in and going out at exactly the same rate, our dynamic equilibrium. So there is no net change in the size of the cell. Then we have hypertonic, hyper. are wrinkled like a raisin. Look at this. It is shriveled. It is wrinkled. It is so sad. Okay. That is called cremation. And that's when we're losing water. Now look at our plant cells. Plant cells are crazy. Plant cells have that cell wall, which is going to, you know, change the way that all this happens a little bit. Plant cells are greedy and they want a lot of water. So look at this under hypotonic, big fat. Oh, our cells are like busting and dead. Plant cells are like, hey man, this is where I want to be. Why? Because they have this large central vacuole that needs to be like essentially applying, it needs to be so full that it's like squishing out, pushing, like it's trying to break free of that cell, pushing against that rigid cell wall. Remember, there's a cell membrane, but then the cell wall is outside of that. Okay, so this is called turgid. It's also called turgor pressure. This is where a cell wants to be. Okay, a cell is flaccid in an isotonic solution. It's a little bit wilty, not like dead, but like a little bit wilty, like a little bit like celery that's like not real crisp, but like you'd eat it in a pinch, okay? Water is going in and out at an equal rate in this case. And then in a hypertonic solution, this is called plasmolysis. So this cell has been plasmolyzed. So you see like these little like, I don't know, like bubbly, like wavy. The cytosol is literally pulling away from the cell membrane. Like it's literally like shrinking on the inside, but like the cell wall makes it look like, oh no, we're still fine. It's not fine. Okay. This is your wilted flowers, your celery that wiggles when you wave it back and forth. Like this is nasty. Okay. This is plant cells that are not happy. Okay. Plant cells are happy in a hypotonic environment. So that central vacuole is full and it's literally pushing outwards. applying pressure to that cell membrane. Okay, so these different types of tonicities, these hypertonic hypotonic environments that are created from osmosis can have like some issues for some organisms. This is when that like contractile vacuole comes into play that we talked about in our last section. So the contractile vacuole's job is to essentially hold on to the excess water because these organisms live in hypo- hypotonic environments, hypo, hippo, big, fat, oh, where's the water going? It's going into the cell, okay? But now, if this cell is special, like this protist, this paramecium, okay, that has its contractile vacuole, it's going to have all that extra water that's rushing into the cell stored in the contractile vacuole, and then it's going to pump it right back out again, okay? And notice that it says that the protist is hypertonic to its pond water. If the cell is hypertonic, then the water has to be hypotonic. Okay. They have to be opposites. That's a picture of a contractile vacuole. Okay. So cell walls and this whole turgid and flaccid and... Plasma lysis and all of that. I just explained that but I explained it up at the picture I just forgot that it came later. So like this is what I just said, okay So that's the same picture again that we just talked about again plant cells are happy here and our cells are happy in Isotonic eyes so healthy. Okay. Our next type of passive transport is facilitated diffusion which means aided diffusion, the movement of molecules from high concentration to low concentration with a helper. Who is the helper? The helper is a protein. Okay, so transport proteins are going to help speed the passive movement of molecules across the plasma membrane. So we have channel proteins that are going to provide like a walkway, an alleyway that allows specific molecules or ions to just go in and out of the membrane as needed with their concentration gradient. OK, so an example would be aquaporins, which we talked about previously. That's facilitated diffusion of water. OK, and the movement of water through a selectively permeable membrane is still osmosis. OK, our ion channels, these are going to open or close in response to a stimulus. So that would be like a gated channel. So if we have a charge that's built up on one end of the cell membrane and we need to send some of these charged molecules back through. It's going to trigger this gate to open and allow molecules to pass back through into the cell membrane or vice versa, just depending on the circumstance. So here we have a channel protein up at the top, and it's going to be allowing molecules to move through the cell membrane through that walkway. It's providing a walkway in and out of the cell, moving from high concentration to low concentration. Okay, whereas in the bottom we have a carrier protein. Now, this is not the same as a channel. protein because this literally has to change shape and this one is going to be a different reaction and it's actually called active transport. Okay. So we do have, oh, okay. So this one in particular is not going to actually be using energy. I lied to you just now. A lot of carrier proteins do. There are a few that do not use any energy. This is one of those few without labels here. You know, it's a little, a little ambiguous, but that's fine. Okay, so carrier proteins can undergo shape changes in order to open from one side of the membrane to the other side of the membrane to transport substances across. And what causes them to change sides or flip is when the solute binds to the actual receptor itself. Okay, now that's the end of our passive transport. So active transport, think about being active. active, you're utilizing a lot of what? Energy. And energy in a cell is called ATP. Active transport literally contains all three of those letters. It's nice how that works. ATP, active transport, okay? Uses energy to move solutes, stuff, against its concentration gradient. It's like going against gravity from low to high. It's against the gradient, okay? So while facilitated diffusion is going to move things down the concentration gradient, okay, we're going to have other types of transport that are going to move things against their gradient. Okay, active transport moves things against their concentration gradient using ATP. So our first example here is going to be a sodium-potassium pump, creatively named. What do you think it does? You're right, it moves sodium-potassium around the cell, in and out of the cell. Okay, let's look at the pictures. So this is the overall process, one, two, three, four, and so on. Okay, it kind of goes around like in a circle, like a clockwise circle for you. So let's look at each of these steps. So in a sodium potassium pump that uses energy in order to transport sodium potassium in and out of the cell, here's what happens. Cytoplasmic sodium is going to bind to the pump, which causes the affinity for sodium. I'm sorry, the affinity for sodium is high when the protein has this particular shape. Okay, so sodium is like, hey man, I'm going to come bind to you and three of them attach. Okay, then we have the binding of that sodium is going to cause ATP to come along and phosphorylate or donate a phosphate to this protein. And ATP is our energy. When you take off a phosphate, you're releasing the energy. So what you're doing is you're attaching the terminal phosphate here to our protein. Let's see what happens next. When you have this phosphate attached, it causes the protein to change shapes. Notice it's open down here, and then as soon as you add that phosphate, it opens up. Why? Because what you did is you just took those sodiums from inside the cell and you put them outside the cell from low concentration to high concentration because this is active transport. Okay, so phosphorylation leads to the change in the protein shape. Yep, that's correct. Now it's open up. reducing its affinity for the sodium, so the sodium will pop off. Now it's open to the outside, okay? The new shape has a high affinity for K+, which is potassium. Okay, so potassium from outside is going to bind to our sodium potassium pump, and it's going to release this phosphate. Now, PI is inorganic phosphate. Why is it called inorganic phosphate? Because it doesn't contain carbon to hydrogen bonds. It's PO4 minus. Okay, there's no C or H's in there. That's what that means, okay? So here you have your pump. and you're releasing, you're dephosphorylating, you're removing the phosphate group, and let's see what happens. Just like that, it flips back down. So it's going to be open to the cytoplasmic side of the cell membrane now to release these potassiums. So the loss of the phosphate causes the protein to move back to its original shape, which has a lower affinity for our potassium. Potassium pops off. And now in this shape, we're back to step number one, where we have a very high affinity for our sodium. And the cycle will just repeat itself. OK, this is an example of something called an antiporter because they're going in opposite directions. Sodium goes out, potassium comes in. Out and in are opposites. It's called an antiporter because it is a protein that's moving molecules in opposite directions. So just to give you a visual side-by-side, you have passive transport and active transport here. Diffusion is when you just have small non-polar molecules that can just squeeze themselves in between the phospholipids and just like go into the cell, high to low concentration. Passive transport is also about facilitated diffusion, also osmosis, which we talked about with the aquaporins and all of that, okay? But our facilitated diffusion here is when we have molecules that are moving from high concentration to low concentration with the help of a protein. In this case, we have a channel protein and we have a carrier protein. Now notice that there is no ATP involved anywhere on here. We're going from high to low concentration. Over here, active transport, ATP. Active transport requires ATP to move molecules against their concentration gradient. In this case, we have a high concentration of cubes here. And it looks like we're going to be moving them up here where there's only two. From high concentration to low concentration. Okay, high and low doesn't mean up and down. This is where we have more of the squares, and then we're going to be moving them here, which means less of the squares. That's high concentration to low concentration requires the input of ATP, and that's active transport. Okay, we also can move things called ions, so like charged molecules, in and out of the membrane in order to maintain the membrane potential. So the membrane potential is essentially like... Think about like the charges that are on the inside and outside. It's like it's the voltage that's created by the differences in the distribution of positive and negative charges. So we just talked about a sodium potassium bump, in which case both of the molecules are positively charged ions. And if we're moving three out and two in, we're obviously going to be changing the voltage. We're going to be changing the charges on the inside and outside of the cell. Okay, and so we rely on the membrane potential in order to help us move things in and out of the cell. So two combined forces collectively called our electrochemical gradient drive the diffusion of ions across the membrane. So ions are our charged particles. So you have the chemical force, which is the ions concentration gradient. So like the sodium, potassium, where do you have more, where do you have less? That's the concentration gradient. Okay, and then you also have the electrical force, so the effect on the membrane potential on the ions movement. So what is its charge, positive or negative? Is it like plus one, plus two, so on? Okay, so these two combined are electrochemical gradients that help to drive the diffusion of ions or charged molecules across the membrane. Because remember, opposites attract. So like if you have something that's positively charged, it wants to go towards the negative side of the membrane just naturally. Okay, so an electrogenic pump is a transport protein that generates a voltage across a membrane. So this is the movement of our ions. Okay, a sodium potassium pump is a major electrogenic pump in our animal cells. And we just talked about how they function and what they do. Okay, the main electrogenic pump of plants, fungi, and bacteria is called a proton pump. And this moves protons, so like hydrogen, like plus, right, H plus. Okay, and this helps to store energy for cellular work because as these molecules then like come back down, as you're pumping them like let's say out of a cell, well, you're creating a large positive charge outside the cell and you would have a negative charge inside the cell. So naturally the positives want to go towards the negatives. As they diffuse back through the membrane, you're able to harness that as like cellular energy in order to do work in the cell. So here's an example of that. You have largely like a negative charge on the inside of the cell because there's a lot of proteins that have like negative charges. And you have a proton pump that's moving these protons out of the cell. Okay, well, eventually over time, you have a large concentration of our pluses out here. So this is very positively charged. Later, it will eventually want to diffuse back towards our negative charges. And that helps with the like electrochemical gradient to help move things to desired locations. So we talked about an antiporter previously. So a cotransporter is when we have active transport of molecules indirectly drives the transport of another thing. So like when you have plant cells that are using the gradient of our hydrogen ions generated by the proton pumps, it's also that's active transport. It's going to help to incorporate nutrients into the cell as well. So here this is actually considered like secondary transport because you have primary active transport up here where you have our hydrogen that's pumping out through the proton pump. And like I said, as it's coming back down, you're also going to be grabbing on to like sucrose molecules here and bringing them into this sucrose and hydrogen, sorry, co-transporter. So co-transporter is like a it's like a sim porter, S-Y-M porter. Okay, that means together. So these things are coming back down together. So essentially, this is an example of primary active transport. And this is an example of secondary active transport, because this is not utilize ATP, but ATP needed to be utilized in order to generate this electric chemical gradient where we have all these positive charges out here in order to bring those hydrogens back in or those protons back in. And we're also just going to like sneak in a sucrose with that. That's the symporter and that's secondary transport. The next thing we're going to talk about is bulk transport of materials across the plasma membrane going through like exocytosis which is like an exit and endocytosis so going into so small solutes and water enter and leave the cell through the lipid bilayer or by means of our transport proteins that we've been talking about but then we have these really large molecules like you know our biomolecules or polysaccharides or large proteins that are like globular functional proteins that have a lot of different chains. Okay, like they're too bulky to just go through the membrane, they'd like rip it open. So they go through something called a vesicular transport, which is the transport through a vesicle. So this is bulk transport and it requires energy. So in exocytosis, transport vesicles, a type of vesicular transport, they're going to migrate to the membrane, fuse with it, and then release their contents to the extracellular surface. A lot of secretory cells. use exocytosis to export products. So like cells where you are manufacturing hormones, that hormones need to be released, like to get into your bloodstream or something, right? They're going to be secretory cells that are going to be using exocytosis in order to get rid of that product to send it somewhere else, right? So it's getting rid of that hormone, it's releasing that hormone, so then the hormone can go on and do its job. Same thing with neurotransmitters and things like that. Okay, endocytosis is exactly the opposite of that. Endocytosis is when the cell is going to take in large amounts of material. It's also a type of vesicular transport because these trans these things are transporting in vesicles. Okay, so endocytosis is the reversal of exocytosis and they involve different types of proteins. And just so you're aware, there's technically a third type of vesicular transport, not endo, not exo, but a combination of the two. It's called transcytosis. So this is like a transmembrane protein would be on either side. So this is a vesicle that's going to go all the way through the membrane. So it will go through endocytosis on one end of the cell. It will endocytose. So it will come in and it's a little vesicle. It's going to like travel its merry way around the cell. And then it will go out the other side of the membrane through exocytosis. So it will go into, across, and out of a cell. And that's called transcytosis, which is the third type of vesicular transport. There are three types of endocytosis specifically, though. We have phagocytosis, penocytosis, and receptor-mediated cytosis. Phagocytosis is utilized by our white blood cells. It's called cellular eating. They would eat bacteria that they find because of the hydrocarbon chains extending from the cell membrane that we talked about for cell-to-cell recognition to be like, hey, buddy, you're not supposed to be here. I'm going to eat you. That's essentially what's happening in phagocytosis or cellular eating. In penocytosis, cells can wrap around like large clumps of different solvents and bring in large amounts of fluid, typically water mixed with other like things that are dissolved in other solutes. And then we have receptor mediated endocytosis, which is triggered when a ligand or some substance binds to a particular receptor that's like a locking key and then it triggers. the cell to bring in that particular particle. So here's a picture of these things. Phagocytosis is when you have some sort of food particle or some sort of foreign invader that's supposed to be eaten and dissolved that your cell literally wraps around it and brings it in. Phenocytosis is cellular drinking. So you have some sort of solution that needs to be brought into the cell. And again, this is vesicular transport. So you have these vesicles that are going to be transporting these things into and around the cell. And then in receptor mediated endocytosis, you have these little Y shaped or V shaped receptors on the outside of the cell. When you have a particular molecule that sticks to it, it's called a ligand and a receptor, L-I-G-A-N-D, ligand, ligand, however you'd like to pronounce it. Okay. The ligand, ligand will bind to the receptor and that will trigger the cell to start invagination. Okay. So it will be creating this little internal budding vesicle. So that's in order to bring in that particular substance. So it's like a chemical reaction that happens here between the ligand and the receptor in order to cause this invagination here. And then you have our little vesicle that's formed. Okay, so that's it. That is our cellular transport little lecture. You have both passive and active transport. Make sure that you go through and you understand the differences between each of them, the examples of each of them, how they work. And then definitely be aware. all of the key players of our cell membrane, how it functions, why it does what it does. Okay. Make sure that you read the textbook to go along with this. This is where we stop. We're not going to go on to cellular signaling. So when you start seeing all that, I think it's like after 5.4, if you're reading the textbook, just like, just stop. Okay. So like, this is where we're going to stop for the exam information. Thanks for listening and have a great day.

Okay, so life at the edge. We're talking about the edge of the cell. We're talking about the plasma membrane.

Now remember that every single cell that has ever been or ever will be a cell has a plasma membrane that separates the inside from the outside. So it keeps the internal environment inside and keeps the external environment outside. Think about a water balloon. It is the squishy balloon part of the water balloon.

It's what separates the cell from its environment. And every single cell has a plasma membrane. Now the plasma membrane is selectively permeable. This means it's choosy.

It gets to decide what things are going to come in and what things are going to go out. Okay, so it's getting to select what it is. permeable to. Okay, so that's where that term comes from.

It's getting to choose what comes in and what goes out in order to keep the cell in homeostasis. That's the fancy word for balance. This is kind of like a molecular structure if you were to look at like a like a filled molecular structure of the cellular membrane.

You'll see that all those like little red and gray or red and white areas, that's water. Okay, so we're going to talk about how the membrane is both hydrophilic and hydrophobic. So you can see that the exterior and the most interior portion of the cell membrane is going to be interacting with water. Okay, and then the innards are not so much unless you're dealing with those blue sections there, which I'm assuming are going to be our aquaporins, which transport water. It's a specific protein that's designed to do that.

Okay, so let's talk about the actual setup. What is this squishy balloon of our cells made of, right? So all plasma membranes are made of phospholipids. That's why the plasma membrane is also called the phospholipid bilayer, okay?

So that makes phospholipids the most abundant lipid in our cell membranes. Well, that makes sense. It's called a what?

It's called a phospholipid bilayer. It is literally two layers of these fancy fats called phospholipids. It's a...

fatty acid, two fatty acid tails with the phosphate group slapped on the top. Okay. And these phospholipids are very specific in structure that I just explained. And it gives them these certain properties and it's called being amphipathic, which means that they have both a hydrophobic, water-fearing, and hydrophilic, water-loving region. So you have, it's kind of like a little lollipop with two tails, two sticks.

So the lollipop part is going to be your phosphate head. We'll look at a picture of this in a minute. Okay, that is the part that is the polar head. So it will interact with water because water is also, you know, polar. Okay, and then the hydrocarbon tails are going to be the hydrophobic region, which will actually form the center of our bilayer membrane, which again, we'll look at some pictures in a minute to make sure you have the full on visual for our cell membrane.

Okay, that is the most like stable boundary that we can have for ourselves because we have aqueous solutions both inside and outside of ourselves. So it's important that we have those phosphate groups that are polar heads that are going to interact with the water while still protecting our hydrocarbon tails that are going to be on the interior of the membrane. So this is the overall structure that I'm talking about.

So you see this like little wavy guy here. It's all these like little gray or white phosphate heads with the two little yellow tails coming off of each one of them. So those are our amphipathic phospholipids. So this is a phospholipid by layer, by for two layers, because you have a layer on the top and you have a layer on the bottom. And they come together and kind of make like a hydrocarbon tail sandwich.

Okay. But you don't just see phospholipids here. You see all kinds of other stuff, right? There's a whole system of things happening here. You have these globular protein kidney bean looking structures.

Okay, those are going to be your proteins. And notice that you have some that are called integral proteins, which are integrated. It's funny how words work together there. Integral proteins that are integrated into the cell membrane. And you also have peripheral proteins that are on the periphery of our cell membrane.

Notice that they are on the interior though. Okay, so you have different proteins that are found there. You also have glyco.

proteins. Glyco is a prefix that means sugar. So these are just sugars that are slapped on top of a protein. So you can see that those are outlined in this picture.

You also see glycolipids. So again, sugars that are attached to lipids. You just see sugars themselves like our carbohydrate chains that are extending from the cellular membrane. You also see these fibers that make up our extracellular matrix, which gets really complicated with all of the things we talked about in our last unit with the microtubules, microfilaments, intermediate filaments, all of those things to help us, you know, maintain structure and help to transport things around our cell and also anchor cells to each other and things like that.

You also have these little guys embedded inside of the membrane itself called cholesterol. So remember that cholesterols are steroids, which... Some people think are proteins, but that's false.

They are specialized lipids. So it's a lipid bilayer. When in doubt, it's a lipid. Okay. So these cholesterols are in there to help prevent the hydrocarbon tails from the phospholipids from packing too tightly together because they have these like big, you know, ring structures.

You also have like the cytoskeleton that you see on the internal side there, the cytoplasmic side of the cell membrane, which again are going to help to... you know, anchor some of our organelles. They're going to offer structural support for our cell as it is, right? So we have a lot of different players here, but notice like out of our biomolecules that we've talked about, there's one that I didn't mention. It's a nucleic acid.

Spoiler alert. Okay. What is a nucleic acid? A nucleic acid is DNA. You're right.

DNA. DNA is where? It's inside the nucleus, right?

It's inside the nucleus, which has nothing to do with the cell membrane. OK, the cell membrane is the thing that interacts with the environment. You do not want your DNA interacting with the environment because it can easily be damaged. And we don't need that because that's the directions to make everything else that that organism needs. So we can't we have to protect that.

It's kept somewhere else. OK, so it's often asked on exams, quizzes and so on, which of the following biomolecules is not present in the cellular membrane? Your nucleic acids, it's going to be nucleic acids, okay? Because that's your DNA.

If your DNA is in your cell membrane, you've got a bigger problem, okay? So let's keep going here. So our membrane proteins, we talked about the lipids being amphipathic. A lot of our membrane proteins are also amphipathic because the ones that, like, span or go all the way through the membrane have to kind of have a hydrophilic and hydrophobic region as well.

So you'll see that the hydrophilic... portion is what's going to protrude or extend beyond the internal side because think about it, the outermost part and the innermost part are going to be interacting with water. So those have to be water loving. Whereas the middle section, like those yellow hydrocarbon tails, those sections are all going to be hydrophobic because they're not interacting with water at all.

Okay, so keep that in mind too. So our phospholipids and our proteins are both amphipathic, which means that they have hydrophobic and hydrophilic regions within the same molecule. Okay.

Our cell membrane is also called, we have like, it follows this rule called the fluid mosaic model. Okay. So what does that even mean? Okay.

So it's fluid. Okay. So that means it's not rigid.

It's flexible. That's how I like to think about it. Mosaic has a whole bunch of different pieces.

Well, look at this. Obviously it has a whole bunch of pieces. Okay. And then a model. Well, that's easy.

It's a model. Okay. So fluid mosaic model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. So essentially, you see this picture here.

These things do not have to stay where they are. They're very flexible. They're very loosey-goosey. They can move around. Okay, our phospholipids can shift around.

The proteins can move laterally, which means side to side. Okay, throughout the membrane. So they're not just stuck in these positions, rigid robots.

Okay, we are not in a cell wall. We are in a cell membrane, which is a lot more fluid and flexible than a cell wall. Okay, so all of these components can kind of move around and do what they need to do and go with the flow.

It's kind of chill, relaxed. Okay, it's not this rigid, like one, two, one, two. It's very fluid.

That's why it's a fluid mosaic, lots of pieces model. Makes sense. Okay, and we have groups of certain proteins or certain lipids that can associate with long lasting interactions or short term interactions, just depending on the different functions that they need to accomplish. So sometimes like especially like our peripheral proteins, you'll see that they move a lot.

Our other integral proteins or transmembrane proteins, they're going to shift through the cell membrane as well. And then when we talk about like vesicular transport, like obviously, we're going to be introducing new components to our membrane, of course, still dealing with phospholipid bilayers, but we're going to be introducing possibly new receptors or more phospholipids and things like that the membrane is constantly changing. Okay, so that's Keep in mind all of that when you think about the fluid mosaic model.

So here's a closer up image of our phospholipid bilayer here. This is a much simpler structure because it doesn't include all of the rest of the mosaic. It doesn't include the proteins and the carbs and all of that. So here you can get a better image of just a phospholipid bilayer and where exactly our hydrophilic and hydrophobic portions are. Remember that our phospholipids have a phosphate head that is a polar phosphate head so it will interact with water which makes it hydrophilic.

Whereas the hydrocarbon tails, which are like the yellowy orangey structures here, are literally just made of hydrogen and carbon. They are hydrophobic. So that would be the area that does not include water.

It is the interior of our cell membrane or a plasma membrane. So this is what I was talking about, the fluidity of our membrane. So a lot of the lipids and some of the proteins can move laterally.

So like side to side. OK. So the lateral movement of our phospholipids is really rapid. They're kind of like always like in flux. They're always moving around.

Proteins are a lot bigger, okay? And there's phospholipids in between all of our proteins. So that movement is a whole lot slower. Think about these little tiny, tiny phospholipids that are just flipping around doing whatever they need to do, okay? And then our proteins that are much larger are going to have, it's kind of like trying to walk through like a crowded room.

Like they're just bumping into all these other phospholipids. So their movement is going to be slowed down a lot. And some of our proteins move in a directed manner while others are more anchored in place.

And this has to do with like the extracellular matrix and also the cytoskeleton on the internal side of our membranes. Sometimes you have proteins that are anchored in certain positions and sometimes you have proteins that are moving. That's more of like our peripheral proteins and things like that.

They have directed movement that says you move from point A to point B and then you stop and go back again. Like it's something that is controlled in the cell. We're also going to talk about these little guys called cholesterol and how they work with our different temperatures and the fluidity of our membrane.

So as temperatures cool, just in general, you know that that kind of slows things down. OK, membranes switch from a fluid state to a solid state. You know that you don't want your cells to be frozen.

Right. You don't want the membrane to become a complete solid. OK.

So the temperature at which the membrane solidifies depends on the types of liquids, both inside the cell, outside the cell, and anything that might be present within the cell membrane itself, just depending on the organism. The membrane remains fluid to a lower temperature if it's rich in phospholipids with unsaturated hydrocarbon tails. Now remember, unsaturated means that that lipid does not have all of the hydrogen it possibly can. This kind of lipid has our double bonds, our carbon to carbon double bonds. So it's bent, which prevents them from packing very tightly together.

So let's look at this point again. A membrane remains fluid to a lower temperature if it is rich, so has a lot of phosphates, I'm sorry, phospholipids with unsaturated hydrocarbon tails. If they're all bent and funky angles and everything, you can't pack them all together.

That's why like olive oil is a liquid at room temperature. Also, if you put it in your fridge, it's not going to become like a solid. It's still going to kind of be like a little bit more of like a gel, right?

So as you cool down cells that have a large concentration of unsaturated hydrocarbon tails, the tails still can't pack very tightly together. So it's going to maintain fluidity, which is a good thing for that cell in order to maintain homeostasis with its environment. The whole point of having a cell membrane is to interact with the exterior of the cell, right?

You control what comes in and what goes out of the cell. I need to bring in food particles. I need to get rid of waste particles.

These things are important to keep a cell alive and the cooler temperatures. You know, a cell still needs to live. Right. So it's important to maintain the integrity of the membrane in various temperatures. Membranes must be fluid to work properly.

That's what I was just saying. And it's comparing salad oil. Think about it like olive oil. Right. Which is what I just explained.

So this is another image just to kind of like show you what I was just talking about. The more unsaturated hydrocarbon tails you have, the more spaced out your phospholipids, which just means that you have more fluidity, okay? So you have a very fluid membrane up at the top, and then you have a more viscous, which means that everything is packed a lot tighter together.

And then you have these little guys called cholesterol that we threw into the mix. Now remember, I said cholesterol are a fancy type of lipid. It is called a steroid, okay?

So cholesterol reduces the membrane fluidity at moderate temperatures. But, but, right, it reduces the membrane fluidity at moderate temperatures, comma, however, right, at low temperatures, it's going to prevent the membrane from being solidified, which is very important for life. Because if your membrane is solid, you're probably not going to be alive for very long.

Okay. So that's why cholesterol, I always say that they reduce membrane fluidity, or I'm sorry, that they increase membrane fluidity. Well, it's true, especially at low temperatures. Okay, but here it's saying that at moderate temperatures, it reduces the fluidity.

Well, right, the phospholipids can't move around as easily if you have this cholesterol in there. However, the cholesterol is vital to make sure that your cells function at cooler temperatures. Okay, so that's what I was just kind of explaining there.

So we also have variations in the lipid composition of our cell membrane, just depending on the environment of the organism that the cell is present in. So like I said, cholesterol is also a lipid, fossil lipids are lipids, so you're going to have different concentrations of these two different lipids and among other types of like glycolipids and things like that that are present in the cell membrane, depending on the type of organism and depending on the environment that it lives in. The ability to change the lipid composition in response to a temperature change has evolved in organisms over time.

So this is something that we have evolved over time. So there's organisms called psychrophiles that live like inside of ice caps. There are these little tiny bacteria that live inside of ice caps. They have a whole boatload of cholesterol. OK, that is an adaptation for that specific environment.

And that is a great example of evolution if I've ever seen one. Okay, so a membrane is a collage of different proteins as well, right? We've been talking about lipids. You have all these different kinds of lipids and they're really important, fantastic, wonderful. Now let's talk about the proteins.

There's a whole bunch of different types of proteins. The main ones are going to be your integral proteins and your peripheral proteins. They often work together and so they're kind of clustered together and they're embedded in the fluid matrix of the lipid bilayer.

Or like I said, in the case of peripheral proteins, they're kind of like rolling along the internal surface. So these are the two proteins I was just talking about. So the integral proteins, remember that they have both hydrophilic and hydrophobic regions because they span the whole membrane.

They are transmembrane proteins. They go all the way through it. Okay, so when the integral proteins penetrate the hydrophobic interior of the lipid bilayer, they go all the way through it.

Okay, so they span the membrane, and that's why they're called transmembrane proteins because trans just means like across the membrane, and then it's a protein. Okay, so the hydrophobic regions of the integral... proteins consist of one or more stretches of nonpolar, so it's not going to interact with water, amino acids, okay?

These are often alpha helices. Now recall that alpha helices are the secondary structure for proteins, the primary structure being the string of amino acids held together by peptide bonds, okay? So these integral proteins, because they span, they go all the way through the membrane, they have hydrophobic regions. that contain nonpolar amino acids. That makes sense because the polar regions would interact with water.

So the nonpolar regions would be on the interior side of, like in the middle of our bilayer. Our peripheral proteins are loosely bound to the surface of the membrane, and you typically see them on the internal surface, so like on the cytoplasmic side, and literally think about like a log roll. That's kind of what they do. They like roll along the cell surface.

This is an example of a transmembrane protein. So recall the structure of an amino acid that you have an amino group that has a nitrogen in it and you have a carboxylic acid group that has a carbon in it. That's the N-terminus, the nitrogen side with the amino group, and the C-terminus with the carboxylic acid group.

But of course in between the N-terminus and the C-terminus we could have hundreds to thousands of different amino acids that make up this particular protein. But you'll see that there's a whole bunch of alpha helices and a couple beta sheets up there at the top. I don't know if you can see the mouse moving in these recordings but here we go I'm going to try.

These are like the beta sheets here and then here you have the alpha helices that go all the way through the membrane. So these residues here, these amino acids that are present in these regions are going to be our non-polar ones. Whereas the ones on the surface here and the cytoplasmic side here are going to be more polar because they're going to have the potential to interact with water on the inside and outside of our cell.

So in keeping with our theme of proteins here, proteins have a whole bunch of different functions. OK, our membrane proteins can function to do these six different things. They can be transport proteins. They can have enzymatic activity.

They can be attached to our cytoskeleton and the extracellular matrix. They can be involved in cell-to-cell recognition. They can help with intracellular joining.

And they can also help with signal transduction. So let's take a look at those. These are pictures to kind of like, you know, display each of these different characteristics. So transporting molecules, you see passive and active transport there.

Passive would be on this side. Again, I hope that you can see the arrows. with the blue molecules from high concentration to low concentration without the use of ATP. This is a protein channel. Over here you have a carrier protein or also called a solute pump.

You have the input of ATP which is going to tell you that this is active transport moving from a low concentration to a high concentration. You also have enzymes. So you have enzymes that are embedded here that are going to have substrates and products and substrates and products. So it looks like our initial substrate here is this little guy here and then we have a terminal product here. Okay you also have attachments to the cytoskeleton extracellular matrix.

This is in order to keep things anchored. You also have cell to cell recognition. We use this especially in our immune system that our white blood cells can tell what cells make up our bodies versus what cells do not. So for instance you know if you inhale someone's snot with all the bacteria or virus or whatever is in their body and they're sick or something that's disgusting in the first place but that's how it works.

Okay, then your cells would hopefully identify that something has a foreign invader tag, which would be like these carbohydrates that are extending from the cell membrane. If it's something that the other cell would recognize then it's obviously part of that organism. Whereas if it was a foreign invader, it would not recognize this carbohydrate chain and it would probably trigger some sort of phagocytosis in order to destroy it. Okay we also have an intracellular joining so how to connect cells together and then signal transduction. So you have a receptor on the outside of our membrane and you have some sort of signal molecule perhaps like a hormone that's going to dock and bind to the receptor and then cause some sort of like domino effect some sort of chain of reactions to occur within the cell.

So we're going to talk about cell-to-cell recognition first. So the role of our membrane carbohydrates, like I just talked about in this image here with these little green carbohydrates, is for cell-to-cell recognition. So cells recognize each other by binding to the surface molecules, and so the things that are extending from the cell itself, in this case carbohydrates, okay, most of the time it's going to be carbohydrates. So you have like glycolipids and glycoproteins that have these carbohydrate chains that extend.

It's kind of like waving your hand around. as a cell, right? Like, hey, look at me. I'm here.

I'm supposed to be here. I am a self-cell, okay? It's how they identify each other.

So that's on the extracellular matrix. I'm sorry, that's on the extracellular surface of the cell membrane, obviously. So then other cells would like bump into that and be like, oh, hey, what's up? You're supposed to be here.

Cool. Have a nice day. Okay. And then you have your membrane carbohydrates. They're going to be covalently bonded to the lipids to form glycolipids.

Or more commonly, glycoproteins, which we've talked about previously already. Okay. And the carbohydrates on the external or the extracellular side of the plasma membrane vary among species and individuals.

Like I said, if you have something inside of your body that is not part of your body, your cells know it's not supposed to be there. Okay. And that's actually an area of research right now. And so try to come up with like different drugs to help treat like antibiotic resistant bacteria and also develop vaccines and things like that.

So it's a huge topic in research right now. So our membranes have sidedness, right? They have an internal side and external side.

We've been talking about a lot of things that are happening on the external side, like the cell to cell recognition and our transmembrane proteins that span both sides. Okay. So it's an asymmetrical distribution of all these things that we've been talking about. If you click all the way back to the, you know, like second or third slide where I have that picture that has like the the purple proteins and all of that you'll see that it's not even on either side but that's important because like i said with the cell to cell communication and all that you have to have that all on the outside because if it's on the inside then how are the other cells supposed to pick it you know what i mean that just doesn't make any sense okay so it is built that way because we have the er and the golgi apparatus that are helping to like synthesize our proteins and our lipids and then package them and send them wherever they're supposed to go so they get like these little kind of like a zip code like when you send mail it's You put your zip code like I live in Kingwood 77339, right? Like that's going to be Like my zip code it helps narrow down where I am located And so that's what's put on to these glycolipids Glycoproteins and other like transmembrane proteins and everything that get exported to the cellular surface They have these you know little zip codes to kind of identify like oh where are they supposed to go on the inside on the outside?

All the way through the cell membrane. Let me know. Okay, that's how we can label them and it's all of course processed by the ER and the Golgi apparatus. Okay, so here's a picture of that happening. So you can see that you have our ER, okay, where you have these vesicles that are leaving that hold, in this case, it looks like we have some glycoproteins and we have some glycolipids, okay, and then they're going to be packaged and further modified by the Golgi apparatus to be identified as going to the exterior surface.

In this case, we also have that glycoprotein. that is going to be a transmembrane protein here. So the Golgi apparatus is going to modify and like slap those zip codes onto our products here in order to send them where they're supposed to go, just like mail. Okay, so talking about the cell membrane, we've talked about the different components and how they all work together and that it's very fluid and it's a lot of lipids and it's a lot of proteins, but absolutely zero nucleic acids. Okay, so like what's the point of all this?

We've talked about the selectively permeable membrane. The whole point is to regulate what is going in and what is going out of the cell. So it is going to regulate the transport of substances across its own membrane. Okay, so this is where our molecular traffic is happening.

And again, selectively permeable means that it can control, it chooses. These things can come in. These things cannot come in. So we have things that are like hydrophobic, nonpolar molecules, like our hydrocarbon tails of our lipids, okay?

These can dissolve in the lipid bilayer because the lipid bilayer has that interior region that is hydrophobic. So these things can cross the membrane very easily, okay? Things that are polar.

that have these, you know, slightly negative, slightly positive regions that like to be around water cannot just go through the membrane. They need assistance to do that, right? Can't just go through a hydrophobic area if it is polar.

Has to be only non-polar things that can just easily just slip on through the membrane, whereas our polar molecules and larger molecules cannot. So we have things called transport proteins. We said that proteins help us transport things in and out of the cell.

They absolutely do. So transport proteins allow the passage of our hydrophilic substances across the membrane. Because remember, they can't just go straight in between two phospholipids.

It actually has to be transported through a protein because it is hydrophilic. It cannot cross through the hydrophobic region on the inside of our bilayer. Okay.

So some of our transport proteins can be called channel proteins. They can also be called like carrier proteins. Okay.

But these channel proteins have a hydrophilic... channel that allows certain molecules and ions to use that tunnel for transport. Okay.

And our channel proteins that are specifically called aquaporins help to move water because water is a polar substance, right? So we need to have a, um, we need to have a, a channel protein, a transport protein that's going to allow water to move in and out of the cell. Very important.

Water is a huge part of life. Okay. So those particular proteins are called aquaporins.

I think about like water pores because that's what they are. Okay, we have other transport proteins called carrier proteins. These help to shuttle things back and forth across the membrane. And typically, in order to do this, they're going to change shape.

Whereas more of like our channel is just kind of like a direct route. It's just like a slip and slide in and out. Okay. Whereas our carrier proteins physically change shape, and they typically require the input of energy.

Okay, so we're going to talk about passive transport first. So passive transport is the diffusion of a substance across a membrane with no energy. So our cellular energy is called ATP. So passive transport does not use any ATP, okay?

So an example here is diffusion. So the diffusion is the tendency of molecules to spread out evenly into an available space. For instance, I stand at the front of the classroom, I spray a whole bunch of scented Lysol because some of y'all stanky after gym class, if you know what I mean.

Okay, I stand at the front of the room and I hose the ish out of my classroom. The people sitting right in front of me are going to be coughing and gagging. Sorry, front row.

Right. But eventually, all the way in the back, those people will finally be able to smell it, you know, maybe like 10, 15 minutes later. Okay, that's diffusion. And diffusion is passive transport of molecules from an area of high concentration, which is coming out of the Lysol nozzle. to low concentration, which would be all the way in the back of my room where I was not standing.

It's the furthest point away from me. Okay, that would be a very low concentration of whatever I'm spraying. Okay, so there's an example of diffusion for you.

Okay, so all these molecules are moving around randomly. Okay, but remember that temperature affects diffusion. Okay, so as you increase the temperature, the molecules are going to be moving faster, which will increase the rate of diffusion. So think about organisms that live in hotter or colder environments and how that would impact them, right?

Okay, um, so we have something called a dynamic equilibrium, which is what we're going to be focusing on here. So dynamic equilibrium is when you have like obviously equilibrium. It means equal, it means even, right?

But a dynamic equilibrium is when you have like the same concentration on the inside of the cell and the outside of the cell. But it doesn't mean that everything stops moving like oh, it's even now time out stop everyone holds still. That's not what that means, okay? You have things that are constantly moving in and out of the membrane and they're always in motion, but it's happening at about the same rate.

So then the concentration doesn't change. If I have 10 molecules of, you know, oxygen on the inside of the cell and 10 molecules on the outside of the cell, that doesn't just mean, all right, time out, oxygen, stop doing your thing. Like you just, just stay there. Everyone holds still. We're done.

We reached equilibrium. We're finished. Wash our hands of it. We're done.

That's not what that means. Okay, so the oxygen is still going to be coming and going, but it's going to be doing that at an even rate. So it's not like suddenly we have 20 molecules on one side and zero on the other. Okay, it's going to be going in and out at the same rate.

That's what our dynamic equilibrium is. Okay, so here's an example of this diffusion that we just talked about. So we have water and we have molecules of dye.

And you'll notice that there is a selectively permeable membrane, the dotted line. Okay. So these molecules of dye are obviously put in here on the left-hand side, and they're going to bounce around, and they're going to bounce around, and they're going to bounce around. So eventually a couple of them are going to bounce around over here, and then they're going to keep bouncing around, and a couple more are going to get pushed over here.

But does that mean that when they reach an equal equilibrium, an equal amount on either side of this foam membrane here, does that mean that the molecule is like, oh, stop, everyone, we're here, you have arrived. No, they're going to keep bouncing around and doing their thing because they're molecules. They don't got no laws.

Okay. They're going to keep doing their thing. They're going to be bouncing around.

Okay. And they're going to be moving in and out back and forth. And like, if like one or two extra are on one side of the membrane, it's not going to, it's okay because it's going to equal itself back out. It will reach that dynamic equilibrium again.

Okay. Same thing is happening down here, but like this one's a little bit fancy. It has both yellow and purple molecules in it. So it's showing you that the direction of diffusion is still from left to right for a little yellowy orangey guys. Our purple guys are going to be moving from the right side to the left side.

You'll notice that, again, they do reach an equilibrium, but that does not mean that it stops. That's one huge misconception in biology as a whole. Okay, these molecules are still going to be moving in and out. And if it's unequal for like a little bit of time, that's okay. It's going to equal back out again.

Like that's just life, literally. Okay, so substances tend to diffuse down or with their concentration gradient. from where there is more concentration to less concentration.

So where we have a high amount of that stuff, whatever the stuff is that is diffusing, to where we have very little or none of that stuff that is diffusing. Okay, so no work has to be done. No ATP needs to be hydrolyzed.

There is no energy here for substances to move down their concentration gradient. It just naturally flows that way. Think about a bowling ball. You push it up a hill, okay, that's going to require energy.

You get to the top and let it roll down, there's no energy there. That is passive, okay? We're letting it roll at this point.

Okay, so that is what passive transport is because there is no energy utilized by the cell in order to make this reaction happen. Our next kind of passive transport is going to be osmosis. So osmosis is the diffusion of water and specifically water.

If it says water, it's got to be talking about osmosis because that's how it happens. Okay. Please don't bring up osmosis Jones. It's like a triggering point for me.

And everyone's like, let's watch that movie. It's no, isn't it about some kid getting sick, which is not about the movement of water in and out of cells. Anyway, osmosis is the passive transport of water through a selectively permeable membrane.

And again, it's still going to follow the same directions. It's going to be from a high concentration of water to a low concentration of water. Okay, so it will diffuse through a membrane from where you have a lot of water to where you have less water.

Okay, so in this case here, this is called a U-tube. And why is it called a U-tube? Because scientists are creative. No, I'm just kidding. It's a U.

That's why. Okay, not like YouTube, like the thing you're watching this on. No, it's literally the letter.

Okay, so you have water. both sides of the semi-permeable membrane, but then you also have sugar. Sugar is a solute in this situation.

The water is the solvent, the solute is the sugar, and all together it makes a solution. Okay, so our semi-permeable membrane in the middle is permeable to water but not sugar, which means sugar cannot pass through that membrane. So in order to reach our dynamic equilibrium, water's got to move.

That's called osmosis. Okay, so if you look... at the beginning picture here, we have fewer sugar particles here and we have more sugar particles here. Well if there's fewer sugar particles here that means there's going to be more water here. high concentration of water to a low concentration of water comparatively.

That's why the water goes up. Okay. It's going to move from the left side to the right side. Okay.

I like to think about the solute, whatever it is. It could be sugar. It could be salt.

It could be iodine. It could be anything. Okay.

Whatever the solute is, I think about it as salt because salt water is something easy for everyone's brains to understand. Wherever there's more salt, Also called stuff or solute. Whatever it is. Pretend it's salt. Wherever there's more salt, salt sucks.

What does it suck? It sucks water towards it. So if there's a lot of, let's call this salt because it's the sugar, it's the solute, it's the stuff. There's more salt on this side.

Salt sucks the water towards it and oh my god, look at that, the water increased on this side. That's how I like to think about it too. So the definition of osmosis says that water moves from an area of high concentration to an area of low concentration. Okay, but I like to think about it like simpler terms.

Where is there more salt? Salt, it sucks the water towards it. Simple.

Okay, very, very simple. So when we're talking about these solutions and the amount of salt, stuff, solute, whatever that they have in them, right? It's called tonicity is what we're really dealing with. So tonicity is the ability of a surrounding solution to change the cell.

It's either going to gain water or lose water. So we're talking about isotonic, hypertonic. These are tonicities, okay? It's the ability of that solution to affect what's going on with that cell water-wise. So in an isotonic solution, very grammatically correct, I'm going to tell you that I so happy.

A cell is so happy when it is in an isotonic solution because the solute concentration is the same on the inside of the cell and the outside of the cell. This means the salt outside and inside is equal. Salt can't suck to one direction here because it's equal. So what does water do? Does it hold still?

No. This is dealing with a dynamic equilibrium. So the water is going to move in and out at an equal rate. And this is where the cells are happy because they're not changing in size. Nothing bad is happening to them.

This is where they would like to function. Okay, dealing with a hypertonic solution. Hyper, give a little kid a lot of sugar, they are hyper, they have higher energy. Hyper is high amount of solute.

Again, I'm gonna use the word salt, because salt water, am I right? It's very simple, okay? In a hypertonic solution, let's say that you have the ocean.

Great, it's salty. You throw some cells into the ocean. Great, hypertonic solution because there is more salt.

inside of the ocean than there is inside of those cells. Higher salt is hypertonic. What's gonna happen to that cell?

Look at the word hyper. This little R right here, you're gonna think wrinkled like a raisin. Now, wrinkled starts with a W, but English is hard, so let's get over it. Hypertonic, R, wrinkled like a raisin, okay? If you're wrinkling up like a raisin, you're shrinking.

Why? Where's the water going? It's going out of you.

The salt. in the ocean is sucking the water out of that cell. Okay.

Like not literally, but we're going to think about it like that because it's very easy. Okay. Hypertonic solutions have a high amount of stuff, salt. Okay. And it's going to suck the water out of a cell.

And if the water is leaving the cell, think about a balloon, a water balloon, you let the water out. Only get what happens to it. It gets smaller. It shrinks called cremation.

Okay. Hypotonic solutions. Hypo, hippo, big fat O. Okay, so this one was R, wrinkled like a raisin, and I'm telling you hypo, hippo, big fat O. What's gonna happen to that cell?

It's gonna get huge because there's more salt on the inside of the cell. All the water is going to go towards that. It's going to get huge.

And in some cases, it will blow up on a cellular level, which is called lysis. Okay, so this is when you have less solute in the surrounding solution. which means that all of the salt is going to be inside of the cell, then the water is going to go towards that salt, which would be inside of the cell.

And as water rushes in, it's going to cause the cell to pop, just like overfilling a water balloon. So those are three different types of solutions and understand it has to be in relation to something. You can't have a beaker of water in front of you and say, that's an isotonic solution. You don't know. Is there a cell in there and you're watching it be happy and healthy and living its best cellular life?

No. you have to have it in relationship to something else. You have to have a cell inside of the solution before you can judge. Is it iso, hyper, hypotonic? It's a relationship between two different solute concentrations, two different solutions.

So please keep that in mind. So animal cells and plant cells behave a little bit differently in these different environments. So we have a hypotonic solution, hypo, hippo, big fat O. What's happening?

Water's going in. Look at our cell. It is very unhappy. It is lysed, which means it is broken open, not like lice.

bug in your hair that makes you itchy. Okay. Isotonic, again, grammatically correct.

Isohealthy. This is where a cell wants to be. Okay.

This is a red blood cell. It's doing its best life over here. This is a normal environment. Okay.

We have water going in and going out at exactly the same rate, our dynamic equilibrium. So there is no net change in the size of the cell. Then we have hypertonic, hyper.

are wrinkled like a raisin. Look at this. It is shriveled.

It is wrinkled. It is so sad. Okay.

That is called cremation. And that's when we're losing water. Now look at our plant cells. Plant cells are crazy.

Plant cells have that cell wall, which is going to, you know, change the way that all this happens a little bit. Plant cells are greedy and they want a lot of water. So look at this under hypotonic, big fat. Oh, our cells are like busting and dead.

Plant cells are like, hey man, this is where I want to be. Why? Because they have this large central vacuole that needs to be like essentially applying, it needs to be so full that it's like squishing out, pushing, like it's trying to break free of that cell, pushing against that rigid cell wall.

Remember, there's a cell membrane, but then the cell wall is outside of that. Okay, so this is called turgid. It's also called turgor pressure. This is where a cell wants to be.

Okay, a cell is flaccid in an isotonic solution. It's a little bit wilty, not like dead, but like a little bit wilty, like a little bit like celery that's like not real crisp, but like you'd eat it in a pinch, okay? Water is going in and out at an equal rate in this case.

And then in a hypertonic solution, this is called plasmolysis. So this cell has been plasmolyzed. So you see like these little like, I don't know, like bubbly, like wavy. The cytosol is literally pulling away from the cell membrane.

Like it's literally like shrinking on the inside, but like the cell wall makes it look like, oh no, we're still fine. It's not fine. Okay. This is your wilted flowers, your celery that wiggles when you wave it back and forth.

Like this is nasty. Okay. This is plant cells that are not happy.

Okay. Plant cells are happy in a hypotonic environment. So that central vacuole is full and it's literally pushing outwards.

applying pressure to that cell membrane. Okay, so these different types of tonicities, these hypertonic hypotonic environments that are created from osmosis can have like some issues for some organisms. This is when that like contractile vacuole comes into play that we talked about in our last section. So the contractile vacuole's job is to essentially hold on to the excess water because these organisms live in hypo- hypotonic environments, hypo, hippo, big, fat, oh, where's the water going? It's going into the cell, okay?

But now, if this cell is special, like this protist, this paramecium, okay, that has its contractile vacuole, it's going to have all that extra water that's rushing into the cell stored in the contractile vacuole, and then it's going to pump it right back out again, okay? And notice that it says that the protist is hypertonic to its pond water. If the cell is hypertonic, then the water has to be hypotonic. Okay.

They have to be opposites. That's a picture of a contractile vacuole. Okay.

So cell walls and this whole turgid and flaccid and... Plasma lysis and all of that. I just explained that but I explained it up at the picture I just forgot that it came later. So like this is what I just said, okay So that's the same picture again that we just talked about again plant cells are happy here and our cells are happy in Isotonic eyes so healthy. Okay.

Our next type of passive transport is facilitated diffusion which means aided diffusion, the movement of molecules from high concentration to low concentration with a helper. Who is the helper? The helper is a protein. Okay, so transport proteins are going to help speed the passive movement of molecules across the plasma membrane.

So we have channel proteins that are going to provide like a walkway, an alleyway that allows specific molecules or ions to just go in and out of the membrane as needed with their concentration gradient. OK, so an example would be aquaporins, which we talked about previously. That's facilitated diffusion of water. OK, and the movement of water through a selectively permeable membrane is still osmosis. OK, our ion channels, these are going to open or close in response to a stimulus.

So that would be like a gated channel. So if we have a charge that's built up on one end of the cell membrane and we need to send some of these charged molecules back through. It's going to trigger this gate to open and allow molecules to pass back through into the cell membrane or vice versa, just depending on the circumstance. So here we have a channel protein up at the top, and it's going to be allowing molecules to move through the cell membrane through that walkway.

It's providing a walkway in and out of the cell, moving from high concentration to low concentration. Okay, whereas in the bottom we have a carrier protein. Now, this is not the same as a channel. protein because this literally has to change shape and this one is going to be a different reaction and it's actually called active transport. Okay.

So we do have, oh, okay. So this one in particular is not going to actually be using energy. I lied to you just now. A lot of carrier proteins do.

There are a few that do not use any energy. This is one of those few without labels here. You know, it's a little, a little ambiguous, but that's fine. Okay, so carrier proteins can undergo shape changes in order to open from one side of the membrane to the other side of the membrane to transport substances across.

And what causes them to change sides or flip is when the solute binds to the actual receptor itself. Okay, now that's the end of our passive transport. So active transport, think about being active.

active, you're utilizing a lot of what? Energy. And energy in a cell is called ATP. Active transport literally contains all three of those letters. It's nice how that works.

ATP, active transport, okay? Uses energy to move solutes, stuff, against its concentration gradient. It's like going against gravity from low to high. It's against the gradient, okay?

So while facilitated diffusion is going to move things down the concentration gradient, okay, we're going to have other types of transport that are going to move things against their gradient. Okay, active transport moves things against their concentration gradient using ATP. So our first example here is going to be a sodium-potassium pump, creatively named. What do you think it does? You're right, it moves sodium-potassium around the cell, in and out of the cell.

Okay, let's look at the pictures. So this is the overall process, one, two, three, four, and so on. Okay, it kind of goes around like in a circle, like a clockwise circle for you.

So let's look at each of these steps. So in a sodium potassium pump that uses energy in order to transport sodium potassium in and out of the cell, here's what happens. Cytoplasmic sodium is going to bind to the pump, which causes the affinity for sodium.

I'm sorry, the affinity for sodium is high when the protein has this particular shape. Okay, so sodium is like, hey man, I'm going to come bind to you and three of them attach. Okay, then we have the binding of that sodium is going to cause ATP to come along and phosphorylate or donate a phosphate to this protein. And ATP is our energy.

When you take off a phosphate, you're releasing the energy. So what you're doing is you're attaching the terminal phosphate here to our protein. Let's see what happens next.

When you have this phosphate attached, it causes the protein to change shapes. Notice it's open down here, and then as soon as you add that phosphate, it opens up. Why? Because what you did is you just took those sodiums from inside the cell and you put them outside the cell from low concentration to high concentration because this is active transport.

Okay, so phosphorylation leads to the change in the protein shape. Yep, that's correct. Now it's open up. reducing its affinity for the sodium, so the sodium will pop off. Now it's open to the outside, okay?

The new shape has a high affinity for K+, which is potassium. Okay, so potassium from outside is going to bind to our sodium potassium pump, and it's going to release this phosphate. Now, PI is inorganic phosphate. Why is it called inorganic phosphate? Because it doesn't contain carbon to hydrogen bonds.

It's PO4 minus. Okay, there's no C or H's in there. That's what that means, okay? So here you have your pump. and you're releasing, you're dephosphorylating, you're removing the phosphate group, and let's see what happens.

Just like that, it flips back down. So it's going to be open to the cytoplasmic side of the cell membrane now to release these potassiums. So the loss of the phosphate causes the protein to move back to its original shape, which has a lower affinity for our potassium. Potassium pops off. And now in this shape, we're back to step number one, where we have a very high affinity for our sodium.

And the cycle will just repeat itself. OK, this is an example of something called an antiporter because they're going in opposite directions. Sodium goes out, potassium comes in.

Out and in are opposites. It's called an antiporter because it is a protein that's moving molecules in opposite directions. So just to give you a visual side-by-side, you have passive transport and active transport here.

Diffusion is when you just have small non-polar molecules that can just squeeze themselves in between the phospholipids and just like go into the cell, high to low concentration. Passive transport is also about facilitated diffusion, also osmosis, which we talked about with the aquaporins and all of that, okay? But our facilitated diffusion here is when we have molecules that are moving from high concentration to low concentration with the help of a protein. In this case, we have a channel protein and we have a carrier protein. Now notice that there is no ATP involved anywhere on here.

We're going from high to low concentration. Over here, active transport, ATP. Active transport requires ATP to move molecules against their concentration gradient. In this case, we have a high concentration of cubes here. And it looks like we're going to be moving them up here where there's only two.

From high concentration to low concentration. Okay, high and low doesn't mean up and down. This is where we have more of the squares, and then we're going to be moving them here, which means less of the squares. That's high concentration to low concentration requires the input of ATP, and that's active transport.

Okay, we also can move things called ions, so like charged molecules, in and out of the membrane in order to maintain the membrane potential. So the membrane potential is essentially like... Think about like the charges that are on the inside and outside.

It's like it's the voltage that's created by the differences in the distribution of positive and negative charges. So we just talked about a sodium potassium bump, in which case both of the molecules are positively charged ions. And if we're moving three out and two in, we're obviously going to be changing the voltage. We're going to be changing the charges on the inside and outside of the cell. Okay, and so we rely on the membrane potential in order to help us move things in and out of the cell.

So two combined forces collectively called our electrochemical gradient drive the diffusion of ions across the membrane. So ions are our charged particles. So you have the chemical force, which is the ions concentration gradient. So like the sodium, potassium, where do you have more, where do you have less?

That's the concentration gradient. Okay, and then you also have the electrical force, so the effect on the membrane potential on the ions movement. So what is its charge, positive or negative?

Is it like plus one, plus two, so on? Okay, so these two combined are electrochemical gradients that help to drive the diffusion of ions or charged molecules across the membrane. Because remember, opposites attract. So like if you have something that's positively charged, it wants to go towards the negative side of the membrane just naturally.

Okay, so an electrogenic pump is a transport protein that generates a voltage across a membrane. So this is the movement of our ions. Okay, a sodium potassium pump is a major electrogenic pump in our animal cells. And we just talked about how they function and what they do.

Okay, the main electrogenic pump of plants, fungi, and bacteria is called a proton pump. And this moves protons, so like hydrogen, like plus, right, H plus. Okay, and this helps to store energy for cellular work because as these molecules then like come back down, as you're pumping them like let's say out of a cell, well, you're creating a large positive charge outside the cell and you would have a negative charge inside the cell.

So naturally the positives want to go towards the negatives. As they diffuse back through the membrane, you're able to harness that as like cellular energy in order to do work in the cell. So here's an example of that. You have largely like a negative charge on the inside of the cell because there's a lot of proteins that have like negative charges. And you have a proton pump that's moving these protons out of the cell.

Okay, well, eventually over time, you have a large concentration of our pluses out here. So this is very positively charged. Later, it will eventually want to diffuse back towards our negative charges. And that helps with the like electrochemical gradient to help move things to desired locations. So we talked about an antiporter previously.

So a cotransporter is when we have active transport of molecules indirectly drives the transport of another thing. So like when you have plant cells that are using the gradient of our hydrogen ions generated by the proton pumps, it's also that's active transport. It's going to help to incorporate nutrients into the cell as well.

So here this is actually considered like secondary transport because you have primary active transport up here where you have our hydrogen that's pumping out through the proton pump. And like I said, as it's coming back down, you're also going to be grabbing on to like sucrose molecules here and bringing them into this sucrose and hydrogen, sorry, co-transporter. So co-transporter is like a it's like a sim porter, S-Y-M porter.

Okay, that means together. So these things are coming back down together. So essentially, this is an example of primary active transport.

And this is an example of secondary active transport, because this is not utilize ATP, but ATP needed to be utilized in order to generate this electric chemical gradient where we have all these positive charges out here in order to bring those hydrogens back in or those protons back in. And we're also just going to like sneak in a sucrose with that. That's the symporter and that's secondary transport. The next thing we're going to talk about is bulk transport of materials across the plasma membrane going through like exocytosis which is like an exit and endocytosis so going into so small solutes and water enter and leave the cell through the lipid bilayer or by means of our transport proteins that we've been talking about but then we have these really large molecules like you know our biomolecules or polysaccharides or large proteins that are like globular functional proteins that have a lot of different chains.

Okay, like they're too bulky to just go through the membrane, they'd like rip it open. So they go through something called a vesicular transport, which is the transport through a vesicle. So this is bulk transport and it requires energy. So in exocytosis, transport vesicles, a type of vesicular transport, they're going to migrate to the membrane, fuse with it, and then release their contents to the extracellular surface. A lot of secretory cells.

use exocytosis to export products. So like cells where you are manufacturing hormones, that hormones need to be released, like to get into your bloodstream or something, right? They're going to be secretory cells that are going to be using exocytosis in order to get rid of that product to send it somewhere else, right? So it's getting rid of that hormone, it's releasing that hormone, so then the hormone can go on and do its job. Same thing with neurotransmitters and things like that.

Okay, endocytosis is exactly the opposite of that. Endocytosis is when the cell is going to take in large amounts of material. It's also a type of vesicular transport because these trans these things are transporting in vesicles.

Okay, so endocytosis is the reversal of exocytosis and they involve different types of proteins. And just so you're aware, there's technically a third type of vesicular transport, not endo, not exo, but a combination of the two. It's called transcytosis.

So this is like a transmembrane protein would be on either side. So this is a vesicle that's going to go all the way through the membrane. So it will go through endocytosis on one end of the cell.

It will endocytose. So it will come in and it's a little vesicle. It's going to like travel its merry way around the cell.

And then it will go out the other side of the membrane through exocytosis. So it will go into, across, and out of a cell. And that's called transcytosis, which is the third type of vesicular transport.

There are three types of endocytosis specifically, though. We have phagocytosis, penocytosis, and receptor-mediated cytosis. Phagocytosis is utilized by our white blood cells. It's called cellular eating. They would eat bacteria that they find because of the hydrocarbon chains extending from the cell membrane that we talked about for cell-to-cell recognition to be like, hey, buddy, you're not supposed to be here.

I'm going to eat you. That's essentially what's happening in phagocytosis or cellular eating. In penocytosis, cells can wrap around like large clumps of different solvents and bring in large amounts of fluid, typically water mixed with other like things that are dissolved in other solutes. And then we have receptor mediated endocytosis, which is triggered when a ligand or some substance binds to a particular receptor that's like a locking key and then it triggers. the cell to bring in that particular particle.

So here's a picture of these things. Phagocytosis is when you have some sort of food particle or some sort of foreign invader that's supposed to be eaten and dissolved that your cell literally wraps around it and brings it in. Phenocytosis is cellular drinking.

So you have some sort of solution that needs to be brought into the cell. And again, this is vesicular transport. So you have these vesicles that are going to be transporting these things into and around the cell.

And then in receptor mediated endocytosis, you have these little Y shaped or V shaped receptors on the outside of the cell. When you have a particular molecule that sticks to it, it's called a ligand and a receptor, L-I-G-A-N-D, ligand, ligand, however you'd like to pronounce it. Okay.

The ligand, ligand will bind to the receptor and that will trigger the cell to start invagination. Okay. So it will be creating this little internal budding vesicle.

So that's in order to bring in that particular substance. So it's like a chemical reaction that happens here between the ligand and the receptor in order to cause this invagination here. And then you have our little vesicle that's formed. Okay, so that's it. That is our cellular transport little lecture.

You have both passive and active transport. Make sure that you go through and you understand the differences between each of them, the examples of each of them, how they work. And then definitely be aware.

all of the key players of our cell membrane, how it functions, why it does what it does. Okay. Make sure that you read the textbook to go along with this.

This is where we stop. We're not going to go on to cellular signaling. So when you start seeing all that, I think it's like after 5.4, if you're reading the textbook, just like, just stop.

Okay. So like, this is where we're going to stop for the exam information. Thanks for listening and have a great day.

Transcript for:Overview of Membrane Structure and Function

Transcript for:
Overview of Membrane Structure and Function